Systems and methods for regulating power conversion systems with output detection and synchronized rectifying mechanisms

ABSTRACT

System and method for regulating a power conversion system. A system controller for regulating a power conversion system includes a first controller terminal and a second controller terminal. The system controller is configured to receive at least an input signal at the first controller terminal, and generate a gate drive signal at the second controller terminal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system. The system controller is further configured to, if the input signal is larger than a first threshold, generate the gate drive signal at a first logic level to turn off the transistor.

1. CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 14/542,443, filed Nov. 14, 2014, which is a continuation of U.S. patent application Ser. No. 13/466,808, filed May 8, 2012, which claims priority to Chinese Patent Application No. 201210118202.7, filed Apr. 12, 2012, all of the above-referenced applications being incorporated by reference herein for all purposes.

2. BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods with output detection and synchronized rectifying mechanisms. Merely by way of example, the invention has been applied to a power conversion system. But it would be recognized that the invention has a much broader range of applicability.

FIG. 1 is a simplified diagram showing a conventional flyback power conversion system. The power conversion system 100 includes a primary winding 110, a secondary winding 112, a power switch 120, a current sensing resistor 122, a rectifying diode 124, a capacitor 126, an isolated feedback component 128, and a controller 102. The controller 102 includes an under-voltage-lockout component 104, a pulse-width-modulation generator 106, a gate driver 108, a leading-edge-blanking (LEB) component 116, and an over-current-protection (OCP) component 114. For example, the power switch 120 is a bipolar transistor. In another example, the power switch 120 is a field effect transistor.

The power conversion system 100 implements a transformer including the primary winding 110 and the secondary winding 112 to isolate an AC input voltage 190 on the primary side and an output voltage 192 on the secondary side. The isolated feedback component 128 processes information related to the output voltage 192 and generates a feedback signal 136. The controller 102 receives the feedback signal 136, and generates a gate-drive signal 130 to turn on and off the switch 120 in order to regulate the output voltage 192. For example, the isolated feedback component 128 includes an error amplifier, a compensation network, and an opto-coupler.

Though the fly-back power conversion system 100 can be used for output voltage regulation, the power conversion system 100 often cannot achieve good output current control without additional circuitry of high cost. Moreover, the required output current sensing resistor in the secondary side usually reduces the efficiency of the power conversion system 100.

FIG. 2(A) is a simplified diagram showing another conventional flyback power conversion system. The power conversion system 200 includes a system controller 202, a primary winding 210, a secondary winding 212, an auxiliary winding 214, a power switch 220, a current sensing resistor 230, two rectifying diodes 260 and 262, two capacitors 264 and 266, and two resistors 268 and 270. For example, the power switch 220 is a bipolar transistor. In another example, the power switch 220 is a MOS transistor.

Information related to the output voltage 250 can be extracted through the auxiliary winding 214 in order to regulate the output voltage 250. When the power switch 220 is closed (e.g., on), the energy is stored in the transformer that includes the primary winding 210 and the secondary winding 212. Then, when the power switch 220 is open (e.g., off), the stored energy is released to the secondary side, and the voltage of the auxiliary winding 214 maps the output voltage on the secondary side. The system controller 202 receives a current sensing signal 272 that indicates a primary current 276 flowing through the primary winding 210, and a feedback signal 274 that relates to a demagnetization process of the secondary side. For example, a switching period of the switch 220 includes an on-time period during which the switch 220 is closed (e.g., on) and an off-time period during which the switch 220 is open (e.g., off).

FIG. 2(B) is a simplified conventional timing diagram for the flyback power conversion system 200 that operates in the discontinuous conduction mode (DCM). The waveform 292 represents a voltage 254 of the auxiliary winding 214 as a function of time, and the waveform 294 represents a secondary current 278 that flows through the secondary winding 212 as a function of time.

For example, as shown in FIG. 2(B), a switching period, T_(s) of the switch 220, starts at time t₀ and ends at time t₃, an on-time period, T_(on), starts at the time t₀ and ends at time t₁, a demagnetization period, T_(demag) starts at the time t₁ and ends at time t₂, and an off-time period, T_(off), starts at the time t₁ and ends at the time t₃. In another example, t₀≤t₁≤t₂≤t₃. In DCM, the off-time period, T_(off), is much longer than the demagnetization period, T_(demag).

During the demagnetization period T_(demag), the switch 220 remains open, the primary current 276 keeps at a low value (e.g., approximately zero). The secondary current 278 decreases from a value 296 (e.g., at t₁) as shown by the waveform 294. The demagnetization process ends at the time t₂ when the secondary current 278 has a low value 298 (e.g., approximately zero). The secondary current 278 keeps at the value 298 for the rest of the switching period. A next switching period does not start until a period of time after the completion of the demagnetization process (e.g., at t₃).

As shown in FIG. 1 and FIG. 2(A), the power conversion system 100 and the power conversion system 200 each use a rectifying diode (e.g., the diode 124 in FIG. 1 and the diode 260 in FIG. 2) on the secondary side for rectification. A forward voltage of the rectifying diode is usually in a range of 0.3V-0.8V. Such a forward voltage often results in significant power loss in operation, and thus causes low efficiency of the power conversion system. For example, when a power conversion system has an output level of 5V/1 A, a rectifying diode with a forward voltage of 0.3V-0.4V causes a power loss of about 0.3 W-0.4 W at a full load (e.g., 1 A). The reduction of the system efficiency is about 4%-6%.

In addition, in order for the power conversion system 200 to achieve low standby power consumption, the switching frequency is often kept low to reduce switching loss under no load or light load conditions. However, when the power conversion system 200 changes from no/light load conditions to full load conditions, the output voltage 250 may drop abruptly and such a voltage drop may not be detected by the system controller 202 instantly because the system controller 202 can often detect the output voltage only during a demagnetization process of each switching cycle. Therefore, the dynamic performance of the power conversion system 200 is often unsatisfactory at a low switching frequency under no/light load conditions. For example, the power conversion system 200 has an output level of 5V/1 A and the output capacitor 264 has a capacitance of 1000 μF. Under no/light load conditions, the switching frequency is 1 kHz corresponding to a switching period of 1 ms. If the output load changes from no/light load conditions (e.g., 0 A) to full load conditions (e.g., 1 A), the output voltage 250 drops 1 V (e.g., from 5 V to 4 V), which is often unacceptable in certain applications.

Hence, it is highly desirable to improve techniques for rectification and output detection of a power conversion system.

3. BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods with output detection and synchronized rectifying mechanisms. Merely by way of example, the invention has been applied to a power conversion system. But it would be recognized that the invention has a much broader range of applicability.

According to one embodiment, a system controller for regulating a power conversion system includes a first controller terminal and a second controller terminal. The system controller is configured to receive at least an input signal at the first controller terminal, and generate a gate drive signal at the second controller terminal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system. The system controller is further configured to, if the input signal is larger than a first threshold, generate the gate drive signal at a first logic level to turn off the transistor, and if the input signal changes from a first value larger than a second threshold to a second value smaller than the second threshold, change the gate drive signal from the first logic level to a second logic level to turn on the transistor.

According to another embodiment, a system controller for regulating a power conversion system includes a first controller terminal and a second controller terminal. The system controller is configured to receive at least an input signal at the first controller terminal, the input signal being proportional to an output voltage associated with a secondary winding of the power conversion system, and generate a gate drive signal at the second controller terminal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system. The system controller is further configured to, only if the input signal changes from a first value larger than a first threshold to a second value smaller than the first threshold, generate a pulse of the gate drive signal to turn on the transistor during a pulse period associated with the pulse.

According to yet another embodiment, a system controller for regulating a power conversion system includes a first comparator, a signal detector, and a driving component. The first comparator is configured to receive an input signal and output a first comparison signal based on at least information associated with the input signal. The signal detector is configured to receive the input signal and output a first detection signal based on at least information associated with the input signal. The driving component is configured to output a gate drive signal based on at least information associated with the first comparison signal and the first detection signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system. The comparator is further configured to determine whether the input signal is larger than a first threshold. The signal detector is further configured to determine whether the input signal changes from a first value larger than a second threshold to a second value smaller than the second threshold. The driving component is further configured to, if the first comparison signal indicates the input signal is larger than the first threshold, generate the gate drive signal at a first logic level to turn off the transistor, and if the first detection signal indicates the input signal changes from the first value larger than the second threshold to the second value smaller than the second threshold, change the gate drive signal from the first logic level to a second logic level to turn on the transistor.

In one embodiment, a system controller for regulating a power conversion system includes a comparator, a pulse signal generator, and a driving component. A comparator is configured to receive an input signal and output a comparison signal based on at least information associated with the input signal. The pulse signal generator is configured to receive at least the comparison signal and generate a pulse signal based on at least information associated with the comparison signal. The driving component is configured to receive the pulse signal and generate a gate drive signal based on at least information associated with the pulse signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system. The comparator is further configured to determine whether the input signal is larger than or smaller than a threshold. The pulse signal generator is further configured to, only if the comparison signal indicates the input signal changes from a first value larger than the threshold to a second value smaller than the threshold, generate a first pulse of the pulse signal. The driving component is further configured to, in response to the first pulse of the pulse signal, generate a second pulse of the gate drive signal to turn on the transistor during a pulse period associated with the second pulse.

In another embodiment, a method for regulating a power conversion system includes receiving at least an input signal, processing information associated with the input signal, and generating a gate drive signal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system. The process for generating a gate drive signal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system includes, if the input signal is larger than a first threshold, generating the gate drive signal at a first logic level to turn off the transistor, and if the input signal changes from a first value larger than a second threshold to a second value smaller than the second threshold, changing the gate drive signal from the first logic level to a second logic level to turn on the transistor.

In yet another embodiment, a method for regulating a power conversion system includes receiving at least an input signal, the input signal being proportional to an output voltage associated with a secondary winding of the power conversion system, processing information associated with the input signal, and generating a gate drive signal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system. The process for generating a gate drive signal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system includes, only if the input signal changes from a first value larger than a first threshold to a second value smaller than the first threshold, generating a pulse of the gate drive signal to turn on the transistor during a pulse period associated with the pulse.

In yet another embodiment, a method for regulating a power conversion system includes receiving an input signal, processing information associated with the input signal, and determining whether the input signal is larger than a first threshold. The method further includes generating a comparison signal based on at least information associated with the input signal, determining whether the input signal changes from a first value larger than a second threshold to a second value smaller than the second threshold, and generating a detection signal based on at least information associated with the input signal. In addition, the method includes outputting a gate drive signal based on at least information associated with the comparison signal and the detection signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system. The process for outputting a gate drive signal based on at least information associated with the comparison signal and the detection signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system includes, if the comparison signal indicates the input signal is larger than the first threshold, generating the gate drive signal at a first logic level to turn off the transistor, and if the detection signal indicates the input signal changes from the first value larger than the second threshold to the second value smaller than the second threshold, changing the gate drive signal from the first logic level to a second logic level to turn on the transistor.

In yet another embodiment, a method for regulating a power conversion system includes receiving an input signal, processing information associated with the input signal, and determining whether the input signal is larger than or smaller than a threshold. The method further includes generating a comparison signal based on at least information associated with the first input signal, receiving the comparison signal, and processing information associated with the comparison signal. In addition, the method includes generating a pulse signal based on at least information associated with the comparison signal, receiving the pulse signal, processing information associated with the pulse signal, and generating a gate drive signal based on at least information associated with the pulse signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system. The process for generating a pulse signal based on at least information associated with the comparison signal includes, only if the comparison signal indicates the input signal changes from a first value larger than the threshold to a second value smaller than the threshold, generating a first pulse of the pulse signal. The process for generating a gate drive signal based on at least information associated with the pulse signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system includes, in response to the first pulse of the pulse signal, generate a second pulse of the gate drive signal to turn on the transistor during a pulse period associated with the second pulse.

Depending upon embodiment, one or more of these benefits may be achieved. These benefits and various additional objects, features and advantages of the present invention can be fully appreciated with reference to the detailed description and accompanying drawings that follow.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram showing a conventional flyback power conversion system.

FIG. 2(A) is a simplified diagram showing another conventional flyback power conversion system.

FIG. 2(B) is a simplified conventional timing diagram for the flyback power conversion system as shown in FIG. 2(A) that operates in the discontinuous conduction mode (DCM).

FIG. 3(A) is a simplified diagram showing a power conversion system with a rectifying circuit according to an embodiment of the present invention.

FIG. 3(B) is a simplified diagram showing a power conversion system with a rectifying circuit according to another embodiment of the present invention.

FIG. 4 is a simplified timing diagram for the power conversion system as shown in FIG. 3(A) operating in the discontinuous conduction mode (DCM) according to an embodiment of the present invention.

FIG. 5 is a simplified diagram showing certain components of the secondary controller as part of the power conversion system as shown in FIG. 3(A) according to an embodiment of the present invention.

FIG. 6 is a simplified timing diagram for the power conversion system as shown in FIG. 3(A) that includes the secondary controller as shown in FIG. 5 and operates in the discontinuous conduction mode (DCM) according to an embodiment of the present invention.

5. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods with output detection and synchronized rectifying mechanisms. Merely by way of example, the invention has been applied to a power conversion system. But it would be recognized that the invention has a much broader range of applicability.

FIG. 3(A) is a simplified diagram showing a power conversion system with a rectifying circuit according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The power conversion system 300 includes a controller 302, a primary winding 304, a secondary winding 306, an auxiliary winding 324, a rectifying circuit 301, a diode 320, a current sensing resistor 328, capacitors 312 and 380, resistors 314, 316, 322 and 326, and a power switch 330. The rectifying circuit 301 includes a secondary controller 308, a resistor 318 and a transistor 310. The secondary controller 308 includes terminals 390, 392, 394, 396 and 398. For example, the transistor 310 is a MOSFET. In another example, the power switch 330 is a transistor.

According to one embodiment, when the power switch 330 is closed (e.g., on), the energy is stored in the transformer that includes the primary winding 304 and the secondary winding 306. For example, when the power switch 330 is open (e.g., off), the stored energy is transferred to the secondary side, and the voltage of the auxiliary winding 324 maps an output voltage 350 on the secondary side. In another example, the controller 302 receives a feedback signal 360 from a voltage divider that includes the resistors 322 and 326 for output voltage regulation. In yet another example, during the process of energy transfer (e.g., a demagnetization process), the transistor 310 is turned on and at least part of a secondary current 352 flows through the transistor 310. In yet another example, a turn-on resistance of the transistor 310 is very small (e.g., in the range of tens of milli-ohms). In yet another example, the voltage drop on the transistor 310 when conducting is much smaller than a voltage drop on a rectifying diode (e.g., the diode 124 or the diode 260), and thus the power loss of the power conversion system 300 is greatly reduced compared with the system 100 or the system 200.

According to another embodiment, at the end of the energy transfer process (e.g., the demagnetization process), the secondary current 352 has a low value (e.g., nearly zero). For example, the transistor 310 is turned off to prevent a residual current flowing from an output terminal 351 to ground through the transistor 310. In another example, the power switch 330 remains off (e.g., open) when the transistor 310 is on. In yet another example, the secondary controller 308 receives a voltage signal 362 (e.g., V_(DR)) indicative of a voltage at a terminal 364 of the transistor 310 (e.g., a drain terminal of the transistor 310), and provides a signal 366 (e.g., at terminal G2) to drive the transistor 310.

As discussed above and further emphasized here, FIG. 3(A) is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the controller 302 and the secondary controller 308 are on different chips. In another example, the secondary controller 308 and the transistor 310 are on different chips which are part of a multi-chip package. In yet another example, the secondary controller 308 and the transistor 310 are integrated on a same chip.

FIG. 3(B) is a simplified diagram showing a power conversion system with a rectifying circuit according to another embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The power conversion system 400 includes a controller 402, a primary winding 404, a secondary winding 406, a first auxiliary winding 424, a second auxiliary winding 425, a rectifying circuit 401, diodes 420 and 474, capacitors 412, 476 and 478, a current sensing resistor 428, resistors 414, 416, 470 and 472, and a power switch 430. The rectifying circuit 401 includes a secondary controller 408, a resistor 418 and a transistor 410. For example, the transistor 410 is a MOSFET. In another example, the power switch 430 is a transistor. In yet another example, the rectifying circuit 401 is the same as the rectifying circuit 301.

According to one embodiment, when the power switch 430 is closed (e.g., on), the energy is stored in the transformer that includes the primary winding 404 and the secondary winding 406. For example, when the power switch 430 is open (e.g., off), the stored energy is transferred to the secondary side, and the voltage of the second auxiliary winding 425 maps an output voltage 450 on the secondary side. In another example, the controller 402 receives a feedback signal 460 from a voltage divider that includes the resistors 470 and 472 for output voltage regulation. In another example, during the process of energy transfer (e.g., a demagnetization process), the transistor 410 is turned on and at least part of a secondary current 452 flows through the transistor 410. In yet another example, a turn-on resistance of the transistor 410 is very small (e.g., in the range of tens of milli-ohms).

According to another embodiment, at the end of the energy transfer process (e.g., the demagnetization process), the secondary current 452 has a low value (e.g., nearly zero). For example, the transistor 410 is turned off to prevent a reversal current from flowing from an output terminal to ground through the transistor 410. In another example, the power switch 430 remains off (e.g., open) when the transistor 410 is on. In yet another example, the secondary controller 408 receives (e.g., at terminal DR) a voltage signal 462 indicative of a voltage at a terminal 464 of the transistor 410 (e.g., a drain terminal of the transistor 410), and provides a signal 466 (e.g., at terminal G2) to drive the transistor 410.

As discussed above and further emphasized here, FIG. 3(B) is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the controller 402 and the secondary controller 408 are on different chips. In another example, the secondary controller 408 and the transistor 410 are on different chips which are part of a multi-chip package. In yet another example, the secondary controller 408 and the transistor 410 are integrated on a same chip.

FIG. 4 is a simplified timing diagram for the power conversion system 300 as shown in FIG. 3(A) operating in the discontinuous conduction mode (DCM) according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the waveform 502 represents the power switch 330 being turned on or off as a function of time, the waveform 504 represents the secondary current 352 as a function of time, and the waveform 506 represents the feedback signal 360 as a function of time. In addition, the waveform 508 represents the voltage signal 362 (e.g., at terminal DR) as a function of time, the waveform 510 represents the signal 366 (e.g., at terminal G2) as a function of time, the waveform 512 represents a channel current 368 that flows through the transistor 310 as a function of time, and the waveform 514 represents a body-diode current 370 that flows through a body diode of the transistor 310 (e.g., a parasitic diode) as a function of time.

For example, a switching period of the switch 330 includes an on-time period during which the switch 330 is closed (e.g., on) and an off-time period during which the switch 330 is open (e.g., off). In another example, as shown in FIG. 4, an on-time period of the switch 330 (e.g., T_(on)) starts at time t₄ and ends at time t₅, and an off-time period of the switch 330 (e.g., T_(off)) starts at the time t₅ and ends at time t₉. A demagnetization period associated with the transformer including the primary winding 304 and the secondary winding 306 (e.g., T_(demag)) starts at the time t₅ and ends at time t₈. In yet another example, t₄≤t₅≤t₆≤t₇≤t₈≤t₉.

According to one embodiment, during the on-time period (e.g., T_(on)), the switch 330 is closed (e.g., being turned on) as shown by the waveform 502, and the energy is stored in the transformer that includes the primary winding 304 and the secondary winding 306. For example, the secondary current 352 has a low value 516 (e.g., nearly zero) as shown by the waveform 504. In another example, the voltage signal 362 (e.g., V_(DR)) received by the secondary controller 308 has a value 518 which is higher than zero (e.g., as shown by the waveform 508). In yet another example, the signal 366 is at a logic low level (e.g., as shown by the waveform 510), and the transistor 310 is off. In yet another example, during the on-time period (e.g., T_(on)), the channel current 368 has a low value 520 (e.g., nearly zero as shown by the waveform 512) and the body-diode current 370 has a low value 522 (e.g., nearly zero as shown by the waveform 514).

According to another embodiment, at the end of the on-time period (e.g., at t₅), the switch 330 is open (e.g., off) as shown by the waveform 502, and the energy is transferred to the secondary side. For example, the secondary current 352 increases from the value 516 to a value 524 (e.g., at t₅) as shown by the waveform 504. In another example, the voltage signal 362 (e.g., V_(DR)) decreases from the value 518 to a value 526 (e.g., as shown by the waveform 508). In yet another example, the value 526 is lower than both a first threshold voltage 528 (e.g., V_(th1)) and a second threshold voltage 530 (e.g., V_(th2)). In yet another example, both the first threshold voltage 528 (e.g., V_(th1)) and the second threshold voltage 530 (e.g., V_(th2)) are lower than a ground voltage 372 (e.g., zero volt). In yet another example, the body diode of the transistor 310 begins to conduct, and the body-diode current 370 increases from the value 522 to a value 529 (e.g., as shown by the waveform 514). Thereafter, the signal 366 changes from the logic low level to a logic high level (e.g., at t₆ as shown by the waveform 510) and the transistor 310 is turned on in certain embodiments. For example, the channel current 368 increases from the value 520 to a value 525 (e.g., at t₆ as shown by the waveform 512). In another example, there is a delay (e.g., T_(d)) between the time at which the voltage signal 362 (e.g., V_(DR)) decreases from the value 518 to a value 526 and the time at which the signal 366 changes from the logic low level to the logic high level. In yet another example, the delay (e.g., T_(d)) is zero.

According to yet another embodiment, during the demagnetization period (e.g., T_(demag)) the switch 330 remains open (e.g., off) as shown by the waveform 502. For example, the secondary current 352 decreases from the value 524 as shown by the waveform 504. In another example, if the voltage signal 362 (e.g., V_(DR)) is larger than the first threshold voltage 528 (e.g., at t₇ as shown by the waveform 508), the signal 366 changes from the logic high level to the logic low level (e.g., as shown by the waveform 510). In yet another example, the voltage signal 362 (e.g., V_(DR)) decreases to become lower than the first threshold voltage 528 again (e.g., at t₈ as shown by the waveform 508). In yet another example, the transistor 310 is turned off, and the channel current 368 decreases to a low value 534 (e.g., nearly zero as shown by the waveform 512). In yet another example, the body-diode current 370 flows through the body diode of the transistor 310, and decreases to a low value (e.g., nearly zero at t₉ as shown by the waveform 514). In yet another example, the demagnetization period ends at the time t₉. In yet another example, immediately after the time t₉, the voltage signal 362 increases as shown by a rising edge in the waveform 508, and such a rising edge, even if detected, is not used for determining the switching frequency of the power conversion system 300 (e.g., the load conditions). In yet another example, the secondary current 352 is equal to a sum of the channel current 368 and the body-diode current 370. Thus, a combination of part of the waveform 512 (e.g., between t₅ and t₉) and part of the waveform 514 (e.g., between t₅ and t₉) is equal to part of the waveform 504 (e.g., between t₅ and t₉) in certain embodiments.

According to yet another embodiment of the present invention, FIG. 4 is a simplified timing diagram for the power conversion system 400 shown in FIG. 3(B) operating in the discontinuous conduction mode (DCM). For example, the waveform 502 represents the power switch 430 being turned on or off as a function of time, the waveform 504 represents the secondary current 452 as a function of time, and the waveform 506 represents the feedback signal 460 as a function of time. In addition, the waveform 508 represents the voltage signal 462 (e.g., at terminal DR) as a function of time, the waveform 510 represents the signal 466 (e.g., at terminal G2) as a function of time, the waveform 512 represents a channel current 468 that flows through the transistor 410 as a function of time, and the waveform 514 represents a body-diode current 480 that flows through a body diode of the transistor 410 (e.g., a parasitic diode) as a function of time.

As discussed above and further emphasized here, FIG. 4 is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the power conversion system 300 shown in FIG. 3(A) or the power conversion system 400 shown in FIG. 3(B) operating in other modes, such as a continuous conduction mode and the critical conduction mode (e.g., the quasi-resonant mode), can also implement the scheme as illustrated in FIG. 4.

In certain embodiments, the scheme as illustrated in FIG. 4 is implemented in the continuous conduction mode. For example, if the secondary controller 308 detects a falling edge of the signal 362 (e.g., V_(DR)), the secondary controller 308 changes the signal 366 to turn on the transistor 310. In another example, the controller 302 turns on the transistor 330 before the demagnetization period ends (e.g., the secondary current 352 being larger than zero), and in response the signal 362 (e.g., V_(DR)) increases. In yet another example, the secondary controller 308 detects a rising edge of the signal 362 and changes the signal 366 to turn off the transistor 310.

FIG. 5 is a simplified diagram showing certain components of the secondary controller 308 as part of the power conversion system 300 according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The secondary controller 308 includes a clamping component 602, an offset component 604, a rising-edge detection component 606, comparators 608 and 624, a falling-edge detection component 610, a timing controller 612, a logic control component 614, a gate driver 616, a light-load detector 618, a signal generator 620, an oscillator 622, an under-voltage-lockout component 628, and a reference-signal generator 626. For example, some components of the secondary controller 308 are used for synchronized rectifying, including the clamping component 602, the offset component 604, the rising-edge detection component 606, the comparator 608, the falling-edge detection component 610, the timing controller 612, the logic control component 614, and the gate driver 616. In another example, certain components of the secondary controller 308 are used for output voltage detection and control, including the light-load detector 618, the signal generator 620, the oscillator 622, the reference-signal generator 626, the logic control component 614, and the gate driver 616. In yet another example, the components of the secondary controller 308 that are used for synchronized rectifying and the components of the secondary controller 308 that are used for output voltage detection and control are integrated on a same chip.

FIG. 6 is a simplified timing diagram for the power conversion system 300 that includes the secondary controller 308 as shown in FIG. 5 and operates in the discontinuous conduction mode (DCM) according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the waveform 702 represents the power switch 330 being turned on or off as a function of time, the waveform 704 represents the feedback signal 360 as a function of time, and the waveform 706 represents the voltage signal 362 (e.g., at terminal 390) as a function of time. In addition, the waveform 708 represents the signal 366 (e.g., at terminal 392) as a function of time, the waveform 710 represents a channel current 368 that flows through the transistor 310 as a function of time, and the waveform 712 represents a voltage signal 388 (e.g., at terminal 398) that indicates the output voltage 350 as a function of time.

According to one embodiment, the clamping component 602 receives the voltage signal 362 (e.g., V_(DR)) from the terminal 390 (e.g., terminal DR). For example, the rising edge detection component 606, the comparator 608 and the falling edge detection component 610 receive a signal 658 which is equal to the voltage signal 362 modified by the offset component 604. In another example, the rising edge detection component 606, the comparator 608 and the falling edge detection component 610 output signals 670, 660 and 650 respectively based on at least information associated with the signal 658. In yet another example, the timing controller 612 receives the signals 670, 660 and 650 and outputs a signal 672 to the logic controller 614 in order to drive the transistor 310. The offset component 604 is omitted in some embodiments.

According to another embodiment, before time t₁₆, the power conversion system 300 is under no/light load conditions and the switching frequency of the system 300 is kept low (e.g., lower than a threshold). For example, during an on-time period (e.g., between time t₁₁ and time t₁₂), the switch 330 is closed (e.g., being turned on) as shown by the waveform 702, and the energy is stored in the transformer that includes the primary winding 304 and the secondary winding 306. In another example, the voltage signal 362 (e.g., at terminal DR) has a value 714 (e.g., as shown by the waveform 706), and is clamped by the clamping component 602. In yet another example, the signal 366 (e.g., at terminal G2) is at a logic low level (e.g., as shown by the waveform 708), and the transistor 310 is off. In yet another example, during the on-time period (e.g., T_(on)), the channel current 368 has a low value 716 (e.g., nearly zero as shown by the waveform 710). In yet another example, the voltage signal 388 (e.g., V_(s)) has a value 718 (e.g., as shown by the waveform 712).

According to yet another embodiment, at the end of the on-time period (e.g., at t₁₂), the switch 330 is open (e.g., off) as shown by the waveform 702, and the energy is transferred to the secondary side. For example, the voltage signal 362 decreases from the value 714 to a value 720 (e.g., as shown by the waveform 706). In yet another example, the value 720 is lower than both a third threshold voltage 722 (e.g., V_(th3)) and a fourth threshold voltage 724 (e.g., V_(th4)). In yet another example, both the third threshold voltage 722 (e.g., V_(th3)) and the fourth threshold voltage 724 (e.g., V_(th4)) are lower than a ground voltage 372. In yet another example, the body diode of the transistor 310 begins to conduct, and the body-diode current 370 increases in magnitude. Thereafter, the signal 366 changes from the logic low level to a logic high level (e.g., at t₁₃ as shown by the waveform 708), and the transistor 310 is turned on in certain embodiments. For example, the third threshold voltage 722 (e.g., V_(th3)) and the fourth threshold voltage 724 (e.g., V_(th4)) are the same as the first threshold voltage 528 and the second threshold voltage 530, respectively.

According to yet another embodiment, when the voltage signal 362 decreases from the value 714 to the value 720 (e.g., as shown by the waveform 706), the falling-edge detection component 610 detects the drop of the voltage signal 362 and changes the signal 650 in order to turn on the transistor 310. For example, in response, the channel current 368 increases from the value 716 to a value 726 (e.g., at t₁₃ as shown by the waveform 710). In another example, a voltage drop between the drain terminal and the source terminal of the transistor 310 is determined based on the following equation: V _(DS) _(_) _(M2) =−I _(sec) ×R _(ds) _(_) _(on)  (Equation 1) where V_(DS) _(_) _(M2) represents the voltage drop between the drain terminal and the source terminal of the transistor 310, I_(sec) represents the secondary current 352, and R_(ds) _(_) _(on) represents a turn-on resistance of the transistor 310.

Because the turn-on resistance of the transistor 310 is very small, the magnitude of the voltage drop between the drain terminal and the source terminal of the transistor 310 is much smaller than a forward voltage of a rectifying diode (e.g., the diode 124 or the diode 260), according to certain embodiments. For example, when the secondary current 352 becomes very small (e.g., approximately zero), the voltage drop between the drain terminal and the source terminal of the transistor 310 becomes very small in magnitude, and the voltage signal 362 is very small in magnitude. In another example, if the signal 658 is larger than the reference signal 652 in magnitude, the comparator 608 changes the signal 660 in order to turn off the transistor 310. In yet another example, the signal 366 changes from the logic high level to the logic low level (e.g., at t₁₄ as shown by the waveform 708) and the transistor 310 is turned off. In yet another example, the body diode of the transistor 310 begins to conduct again, and the body-diode current 370 decreases in magnitude (e.g., eventually to nearly zero at t₁₅). Thus, the energy is completely delivered to the output in some embodiments.

In one embodiment, the secondary controller 308 continuously monitors the output voltage 350 through the signal 388 (e.g., V_(s)). For example, the comparator 624 receives a reference signal 680 and the signal 388 (e.g., V_(s)) and outputs a signal 682. In another example, the light-load detector 618 receives a clock signal from the oscillator 622 and a signal 676 from the timing controller 612. In yet another example, the signal 676 indicates certain switching events (e.g., rising edges or falling edges) in the signal 362. In yet another example, the light-load detector 618 outputs a signal 678 which indicates the switching frequency of the power conversion system 300. In yet another example, the signal generator 620 receives the signal 678 and the signal 682 and outputs a signal 684 to the logic control component 614 in order to affect the status of the transistor 310.

In another embodiment, if the output voltage 350 drops below a threshold level in any conditions, for example, when the output load conditions changes from no/light load conditions to full load conditions (e.g., between t₁₆ and t₁₇), the output voltage 350 decreases (e.g., below a threshold level). For example, if the signal 388 (e.g., V_(s)) changes from a first value larger than the reference signal 680 in magnitude to a second value lower than the reference signal 680 in magnitude (e.g., at t₁₆ as shown by the waveform 712), the comparator 624 generates a pulse in the signal 682 in order to turn on the transistor 310 during a short time period. In some embodiments, if the signal 678 indicates that the power conversion system 300 is under no/light load conditions, the signal generator 620 outputs a pulse in the signal 684, and in response the gate driver 616 generates a pulse 730 in the signal 366 (e.g., as shown by the waveform 708). For example, the signal 362 (e.g., at terminal DR) decreases to a value 728 (e.g., between t₁₆ and t₁₇ as shown by the waveform 706). In yet another example, the transistor 310 is turned on during a pulse period associated with the pulse 730 in the signal 366, and the channel current 368 flows in a different direction (e.g., from the output capacitor 312 through the transistor 310 to ground) as shown by the waveform 710. In yet another example, the feedback signal 360 increases in magnitude, and forms a pulse (e.g., between t₁₆ and t₁₇ as shown by the waveform 704). The controller 302 detects the pulse of the feedback signal 360 and, in response, increases the peak current of the primary winding 304 and the switching frequency in order to deliver more energy to the secondary side according to certain embodiments. For example, the output voltage 350 and the voltage signal 388 increase in magnitude eventually (e.g., at t₁₈ as shown by the waveform 712).

As discussed above and further emphasized here, FIG. 5 and FIG. 6 are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the secondary controller 408 is the same as the secondary controller 308 as shown in FIG. 5.

In certain embodiments, FIG. 6 is a simplified timing diagram for the power conversion system 400 that includes the secondary controller 408 and operates in the discontinuous conduction mode (DCM). For example, the waveform 702 represents the power switch 430 being turned on or off as a function of time, the waveform 704 represents the feedback signal 460 as a function of time, and the waveform 706 represents the voltage signal 462 as a function of time. In addition, the waveform 708 represents the signal 466 as a function of time, the waveform 710 represents a channel current 468 that flows through the transistor 410 as a function of time, and the waveform 712 represents a voltage signal 488 that indicates the output voltage 450 as a function of time.

In some embodiments, the secondary controller 308 as part of the power conversion system 300 or the secondary controller 408 as part of the power conversion system 400 operating in other modes, such as a continuous conduction mode and the critical conduction mode (e.g., the quasi-resonant mode), can also implement the scheme as illustrated in FIG. 5 and FIG. 6.

According to another embodiment, a system controller for regulating a power conversion system includes a first controller terminal and a second controller terminal. The system controller is configured to receive at least an input signal at the first controller terminal, and generate a gate drive signal at the second controller terminal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system. The system controller is further configured to, if the input signal is larger than a first threshold, generate the gate drive signal at a first logic level to turn off the transistor, and if the input signal changes from a first value larger than a second threshold to a second value smaller than the second threshold, change the gate drive signal from the first logic level to a second logic level to turn on the transistor. For example, the system is implemented according to FIG. 3(A), FIG. 3(B), FIG. 4, FIG. 5, and/or FIG. 6.

According to another embodiment, a system controller for regulating a power conversion system includes a first controller terminal and a second controller terminal. The system controller is configured to receive at least an input signal at the first controller terminal, the input signal being proportional to an output voltage associated with a secondary winding of the power conversion system, and generate a gate drive signal at the second controller terminal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system. The system controller is further configured to, only if the input signal changes from a first value larger than a first threshold to a second value smaller than the first threshold, generate a pulse of the gate drive signal to turn on the transistor during a pulse period associated with the pulse. For example, the system is implemented according to at least FIG. 3(A), FIG. 3(B), FIG. 5, and/or FIG. 6.

According to yet another embodiment, a system controller for regulating a power conversion system includes a first comparator, a signal detector, and a driving component. The first comparator is configured to receive an input signal and output a first comparison signal based on at least information associated with the input signal. The signal detector is configured to receive the input signal and output a first detection signal based on at least information associated with the input signal. The driving component is configured to output a gate drive signal based on at least information associated with the first comparison signal and the first detection signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system. The comparator is further configured to determine whether the input signal is larger than a first threshold. The signal detector is further configured to determine whether the input signal changes from a first value larger than a second threshold to a second value smaller than the second threshold. The driving component is further configured to, if the first comparison signal indicates the input signal is larger than the first threshold, generate the gate drive signal at a first logic level to turn off the transistor, and if the first detection signal indicates the input signal changes from the first value larger than the second threshold to the second value smaller than the second threshold, change the gate drive signal from the first logic level to a second logic level to turn on the transistor. For example, the system is implemented according to FIG. 3(A), FIG. 3(B), FIG. 4, FIG. 5, and/or FIG. 6.

In one embodiment, a system controller for regulating a power conversion system includes a comparator, a pulse signal generator, and a driving component. A comparator is configured to receive an input signal and output a comparison signal based on at least information associated with the input signal. The pulse signal generator is configured to receive at least the comparison signal and generate a pulse signal based on at least information associated with the comparison signal. The driving component is configured to receive the pulse signal and generate a gate drive signal based on at least information associated with the pulse signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system. The comparator is further configured to determine whether the input signal is larger than or smaller than a threshold. The pulse signal generator is further configured to, only if the comparison signal indicates the input signal changes from a first value larger than the threshold to a second value smaller than the threshold, generate a first pulse of the pulse signal. The driving component is further configured to, in response to the first pulse of the pulse signal, generate a second pulse of the gate drive signal to turn on the transistor during a pulse period associated with the second pulse. For example, the system is implemented according to at least FIG. 3(A), FIG. 3(B), FIG. 5, and/or FIG. 6.

In another embodiment, a method for regulating a power conversion system includes receiving at least an input signal, processing information associated with the input signal, and generating a gate drive signal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system. The process for generating a gate drive signal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system includes, if the input signal is larger than a first threshold, generating the gate drive signal at a first logic level to turn off the transistor, and if the input signal changes from a first value larger than a second threshold to a second value smaller than the second threshold, changing the gate drive signal from the first logic level to a second logic level to turn on the transistor. For example, the method is implemented according to FIG. 3(A), FIG. 3(B), FIG. 4, FIG. 5, and/or FIG. 6.

In yet another embodiment, a method for regulating a power conversion system includes receiving at least an input signal, the input signal being proportional to an output voltage associated with a secondary winding of the power conversion system, processing information associated with the input signal, and generating a gate drive signal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system. The process for generating a gate drive signal based on at least information associated with the input signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system includes, only if the input signal changes from a first value larger than a first threshold to a second value smaller than the first threshold, generating a pulse of the gate drive signal to turn on the transistor during a pulse period associated with the pulse. For example, the method is implemented according to at least FIG. 3(A), FIG. 3(B), FIG. 5, and/or FIG. 6.

In yet another embodiment, a method for regulating a power conversion system includes receiving an input signal, processing information associated with the input signal, and determining whether the input signal is larger than a first threshold. The method further includes generating a comparison signal based on at least information associated with the input signal, determining whether the input signal changes from a first value larger than a second threshold to a second value smaller than the second threshold, and generating a detection signal based on at least information associated with the input signal. In addition, the method includes outputting a gate drive signal based on at least information associated with the comparison signal and the detection signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system. The process for outputting a gate drive signal based on at least information associated with the comparison signal and the detection signal to turn on or off a transistor in order to affect a current associated with a secondary winding of the power conversion system includes, if the comparison signal indicates the input signal is larger than the first threshold, generating the gate drive signal at a first logic level to turn off the transistor, and if the detection signal indicates the input signal changes from the first value larger than the second threshold to the second value smaller than the second threshold, changing the gate drive signal from the first logic level to a second logic level to turn on the transistor. For example, the method is implemented according to FIG. 3(A), FIG. 3(B), FIG. 4, FIG. 5, and/or FIG. 6.

In yet another embodiment, a method for regulating a power conversion system includes receiving an input signal, processing information associated with the input signal, and determining whether the input signal is larger than or smaller than a threshold. The method further includes generating a comparison signal based on at least information associated with the first input signal, receiving the comparison signal, and processing information associated with the comparison signal. In addition, the method includes generating a pulse signal based on at least information associated with the comparison signal, receiving the pulse signal, processing information associated with the pulse signal, and generating a gate drive signal based on at least information associated with the pulse signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system. The process for generating a pulse signal based on at least information associated with the comparison signal includes, only if the comparison signal indicates the input signal changes from a first value larger than the threshold to a second value smaller than the threshold, generating a first pulse of the pulse signal. The process for generating a gate drive signal based on at least information associated with the pulse signal to turn on or off a transistor in order to affect a current associated with the secondary winding of the power conversion system includes, in response to the first pulse of the pulse signal, generate a second pulse of the gate drive signal to turn on the transistor during a pulse period associated with the second pulse. For example, the method is implemented according to at least FIG. 3(A), FIG. 3(B), FIG. 5, and/or FIG. 6.

For example, some or all components of various embodiments of the present invention each are, individually and/or in combination with at least another component, implemented using one or more software components, one or more hardware components, and/or one or more combinations of software and hardware components. In another example, some or all components of various embodiments of the present invention each are, individually and/or in combination with at least another component, implemented in one or more circuits, such as one or more analog circuits and/or one or more digital circuits. In yet another example, various embodiments and/or examples of the present invention can be combined.

Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims. 

What is claimed is:
 1. A system controller for regulating a power conversion system, the system controller comprising: a comparator configured to receive an input signal and output a comparison signal based at least in part on the input signal; a first signal detector configured to receive the input signal and output a first detection signal based at least in part on the input signal; a second signal detector configured to receive the input signal and output a second detection signal based at least in part on the input signal; and a driver configured to: generate a drive signal based on at least information associated with the comparison signal, the first detection signal and the second detection signal; and output the drive signal to a gate terminal of a transistor to turn on or off the transistor in order to affect a current associated with a secondary winding of the power conversion system, the transistor including the gate terminal, a drain terminal and a source terminal; wherein: the comparator is further configured to determine whether the input signal is larger than a threshold; the first signal detector is further configured to determine whether the input signal includes one or more falling edges; the second signal detector is further configured to determine whether the input signal includes one or more rising edges; the driver is further configured to, under a discontinuous conduction mode of the power conversion system: if the comparison signal indicates the input signal is larger than the threshold, generate the drive signal at a first logic level to turn off the transistor; and if the first detection signal indicates the input signal includes the one or more falling edges, change the drive signal from the first logic level to a second logic level to turn on the transistor; and the driver is further configured to, under a continuous conduction mode of the power conversion system: if the first detection signal indicates the input signal includes the one or more falling edges, change the drive signal from the first logic level to a second logic level to turn on the transistor, and if the second detection signal indicates the input signal includes the one or more rising edges, generate the drive signal at the first logic level to turn off the transistor; and wherein the discontinuous conduction mode of the power conversion system is different from the continuous conduction mode of the power conversion system; wherein: the drain terminal is directly connected to a first winding terminal of the secondary winding, the secondary winding further including a second winding terminal; the first winding terminal of the secondary winding is also directly connected to a first resistor terminal of a resister, the resistor further including a second resistor terminal; the second winding terminal is directly connected to a first capacitor terminal of a capacitor, the capacitor further including a second capacitor terminal; the second capacitor terminal is directly connected to the source terminal; the second capacitor terminal and the source terminal both are biased to a ground voltage; and the second resistor terminal is connected to the comparator, the first signal detector and the second signal detector, directly or indirectly through an offset component.
 2. The system controller of claim 1, and further comprising: a timing controller configured to receive the first comparison signal, the first detection signal and the second detection signal and output a timing signal based at least in part on the first comparison signal, the first detection signal and the second detection signal; and a logic controller configured to receive the timing signal and generate a control signal based at least in part on the timing signal; wherein the driver is further configured to receive the control signal and output the drive signal based at least in part on the control signal.
 3. The system controller of claim 1, and further comprising: a clamper configured to receive a first signal and clamp the first signal; and the offset component configured to receive the first signal and generate the input signal based at least in part on the first signal; wherein the second resistor terminal is connected to the comparator, the first signal detector and the second signal detector indirectly through the offset component.
 4. A method for regulating a power conversion system, the method comprising: receiving an input signal; processing information associated with the input signal; determining whether the input signal is larger than a threshold; generating a comparison signal based at least in part on the input signal and the threshold; determining whether the input signal includes one or more falling edges; generating a first detection signal based at least in part on the input signal; determining whether the input signal includes one or more rising edges; generating a second detection signal based at least in part on the input signal; generating a drive signal based on at least information associated with the comparison signal, the first detection signal and the second detection signal; and outputting the drive signal to a gate terminal of a transistor to turn on or off the transistor in order to affect a current associated with a secondary winding of the power conversion system, the transistor including the gate terminal, a drain terminal and a source terminal; wherein the outputting the drive signal to a gate terminal of a transistor to turn on or off the transistor in order to affect a current associated with a secondary winding of the power conversion system includes: under a discontinuous conduction mode of the power conversion system: if the comparison signal indicates the input signal is larger than the threshold, generating the drive signal at a first logic level to turn off the transistor; and if the first detection signal indicates the input signal includes the one or more falling edges, changing the drive signal from the first logic level to a second logic level to turn on the transistor; under a continuous conduction mode of the power conversion system: if the first detection signal indicates the input signal includes the one or more falling edges, changing the drive signal from the first logic level to a second logic level to turn on the transistor, and if the second detection signal indicates the input signal includes the one or more rising edges, generating the drive signal at the first logic level to turn off the transistor; and wherein the discontinuous conduction mode of the power conversion system is different from the continuous conduction mode of the power conversion system; wherein: the drain terminal is directly connected to a first winding terminal of the secondary winding, the secondary winding further including a second winding terminal; the first winding terminal of the secondary winding is also directly connected to a first resistor terminal of a resister, the resistor further including a second resistor terminal; the second winding terminal is directly connected to a first capacitor terminal of a capacitor, the capacitor further including a second capacitor terminal; the second capacitor terminal is directly connected to the source terminal; and the second capacitor terminal and the source terminal both are biased to a ground voltage. 